This invention relates to a high-purity molybdenum target and a high-purity molybdenum silicide target for use in forming films for electrodes of LSI and also to a process for producing such targets.
The gate electrodes of MOS(metal-oxide semiconductor)LSI have hitherto been made of polycrystalline silicon. However, the recent trend toward higher degrees of integration in the fabrication of the MOSLSI is causing a serious problem of signal delay due to the resistivities of polycrystalline silicon gate electrodes. Meanwhile, in order to facilitate the manufacture of MOS devices by the self-aligning technique, the use of high-melting-point materials is desired for the gate electrodes. Varied attempts have therefore been made to utilize the high-melting-point metals and their silicides of lower resistivities than the polycrystalline silicon in the fabrication of MOS gate electrodes.
For the manufacture of MOS devices the so-called self-aligning method is in use which consists in forming a gate electrode and ionically implanting impurities for doping into the interior of the semiconductor, with the gate electrode itself acting as a mask, so as to define source and drain regions. In that case, a heat treatment at about 1000.degree. C. is required to activate the impurities following the ion implantation. This is the reason why a high melting-point material is to be chosen for forming MOS gate electrodes.
Electrode wiring to be formed on silicon devices must satisfy a number of requirements as follows:
(1) High purity
(2) Low resistivity
(3) Great heat resistance
(4) Good adhesion to the SiO.sub.2 film of the substrate
(5) No reaction with, or diffusion in, the SiO.sub.2 substrate film
(6) High ability of blocking ion implantation
(7) Ability of making stable contact with silicon
However, it is not too much to say that electrode materials that can completely satisfy all these conditions are practically nil.
Therefore, as electrodes that can replace the silicon gate electrodes, that is the electrodes amenable to the self-aligning process, studies on high-melting point metal gate electrodes whose low resistivities are the primary consideration and also studies on silicide electrodes primarily compatible with the silicon gate process have been started. However, the resistivities of silicides are lower than that of polycrystalline silicon by only about one order of magnitude, and their application to MOS memories of 256 kilobits and larger scales is considered questionable.
Among low-resistivity, high-melting-point metals, those which fairly satisfy the aforementioned conditions are molybdenum and tungsten, with melting points above 2000.degree. C. and resistivities below 10 .mu..OMEGA.cm. Of the two, molybdenum is capable of forming a high-quality film near a bulk resistivity (5.7.times.10.sup.-6 .OMEGA.cm). Hence molybdenum is a most promising electrode material for future very-large-scale integrations (VLSI).
Methods of forming molybdenum films are broadly divided into three groups; sputtering, vacuum deposition, and chemical vapor deposition (CVD). Of these, sputtering and vacuum deposition using an electron beam are superior in the quality of resulting film, reproducibility, and adaptability for quantitative production. Sputtering is a technique in which a metal target plate is subjected to argon ion bombardment to release the metal from the surface and release metal is deposited on the substrate disposed opposite to the target plate. Electron-beam vacuum deposition involves the melting of a molybdenum ingot source by an electron beam and effecting the vapor deposition. Thus, the purity of the resulting film is governed by the purity of the target plate or vapor source.
The impurities that influence the MOS device performance are classified into the following three groups:
(1) Alkali metals such as sodium, etc.
(2) Radioactive elements such as uranium, thorium etc.
(3) Heavy metals such as iron etc.
Of these impurities, sodium and other alkali metals move easily through the gate dielectric layer to deteriorate the characteristics of the MOS. Radiation damages by the radioactive elements can fatally affect the operational reliability of the MOS device. Iron and other heavy metals can produce surface states or cause junction leaks. For these reasons, minimizing the contents of these impurities, i.e., alkali metals, radioactive elements, and heavy metals, is a basic requirement for the materials to constitute MOSLSI.
The molybdenum target plates commercially available today are ones formed simply by compacting molybdenum powder, sintering the compact, and then machining the sintered body. The grade being claimed to be the purest is at most 99.9% pure. The purity of the cleanest existing molybdenum vapor source is 99.99%. The materials are usually not analyzed for their sodium and uranium contents. Analyses made by the present inventors have revealed that the commercial target plates contain several parts per million sodium, more than 500 parts per billion uranium, and several ten ppm iron. These contents are greater than in the other MOSLSI constituent materials, such as polycrystalline silicon and SiO.sub.2, by at least two orders of magnitude. This obviously indicates that there is no possibility of successfully incorporating molybdenum gate electrodes to MOSLSI and VLSI unless a molybdenum target plate and a molybdenum vapor source are developed which are refined to higher purity levels by at least two orders of magnitude and reduced in their alkali element and radioactive element contents to be generally as pure as the other constituent materials of MOSLSI.
On the other hand, among the silicides as possible electrode materials, molybdenum silicide is being most widely studied. The molybdenum silicide film is often formed by sputtering. In that case, either a molybdenum silicide target is directly employed or a target combining cut pieces of molybdenum and silicon plates in a mosaic pattern is used. Sometimes both molybdenum and silicon targets are provided and they are simultaneously sputtered. So far as silicon is concerned, grades of very high purities already been developed and the purity of the silica electrode depends solely upon that of the molybdenum used. As stated above, the commercially available clean molybdenum is about 99.9% pure and hence the purity of the resulting molybdenum silicide electrode is at most 99.9%. This has been a major limitation to the use of silicide electrodes for MOSLSI.
Thus, there has been urgent need for developing an exceptionally pure molybdenum sputter source or vapor source material, not less than 99.999% pure with very low alkali element and radioactive element contents to attain favorable performance of the electrode, be it of molybdenum or molybdenum silicide.